Thermo‐Fluorescent Bactericidal Quantum Dots Based Smart Multifunctional Textiles via Molecular Surface Engineering and 3D‐Printed Interlocked Architectures

ABSTRACT

Functional textiles integrating optical activity, environmental responsiveness, and structural adaptability are essential for next‐generation wearable systems. Here, we present a versatile strategy for fabricating thermo‐fluorescent, UV‐protective, and antibacterial smart textiles via carbon dot (CD)/polymer nanocomposite coatings. CDs synthesized hydrothermally exhibited strong excitation‐dependent emission (320–460 nm), a fluorescence lifetime of 4.2 ns, and excellent photostability over 30 days. The CDs formed hydrogen‐bonding interactions within a PVA/quaternized chitosan matrix and were deposited onto textiles using dip‐and‐dry and spray methods, followed by hexadecyltrimethoxysilane modification to impart durable hydrophobicity (water contact angle >100°). The coated fabrics maintained fluorescence under repeated mechanical deformation and showed coating‐cycle‐dependent emission. The functional cotton displayed thermo‐responsive fluorescence (5–100°C), with enhanced emission below 37°C and quenching above this temperature. The textiles showed strong antibacterial activity, with inhibition zones of ∼17 mm (E. coli) and ∼19 mm (B. subtilis). The textiles also demonstrated notable antioxidant performance (DPPH EC₅₀ = 88.6 µg mL^{− 1}; ABTS EC₅₀ = 37.1 µg mL^{− 1}) and excellent UV‐blocking, exceeding a 95% relative increment in UVB blocking at the highest coating level. Integration with digital light processing printing enabled mechanically interlocked hybrid architectures without compromising functionality, establishing a multifunctional platform for advanced smart textile applications.

A versatile approach is presented for fabricating smart multifunctional textiles by integrating thermo‐fluorescent carbon dot/polymer nanocomposite coatings with 3D‐printed interlocked architectures. The fabrics exhibit temperature‐responsive fluorescence, durable hydrophobicity, strong antibacterial and antioxidant activity, and enhanced UV protection. This platform combines optical activity, environmental responsiveness, and structural adaptability, offering a scalable route toward next‐generation wearable and functional textile systems.

1. Introduction

Embedding nanoparticles within macroscopic matrices is a major pursuit in nanotechnology and materials science, as it allows the fabrication of materials with enhanced and multifunctional properties. This method utilizes the combined behavior and distinct properties of nanoparticles, leading to materials with tunable optical, electrical, and mechanical properties that outshine those of the individual components [1]. Fluorescent nanohybrids represent an emerging and highly promising platform for engineering robust opto‑responsive systems with multifunctional performance, high sensitivity, and improved stability. Recent advances demonstrate that these nanostructured hybrids can be tailored to generate distinct visual responses to thermal or mechanical stimuli, enabling real‑time monitoring and advanced sensing capabilities for both biomedical and environmental applications [2, 3]. Thermal‐responsive optical and fluorescent materials have gained a lot of attention because of their ability to convert thermal stimuli into detectable optical signals, allowing for real‐time monitoring of temperature‐dependent activities. The materials provide distinct advantages in sensing, diagnostics, and advanced biomedical applications, where precision thermal feedback is essential. Polydiacetylene and polydimethylsiloxane have been combined to develop stretchable materials that exhibit both thermochromic and thermofluorescent responses within the temperature range of 25–85°C [4]. Wang et al. reported that tetraphenylethene derivatives exhibit simultaneous thermofluorescence and thermochromism within the temperature range of 80–120°C [5]. Importantly, material flexibility and stretchability are essential for applications requiring seamless mechanical adaptability and reliable performance under deformation of soft substrates or matrices. Despite their promising optical and thermal properties, the use of carbon dots in such thermo‐optical systems remains largely unexplored. Compared to conventional materials, the luminescent properties of carbon dots are inherently unique, enabling light emission with high energy efficiency. Consequently, developing a biocompatible nanohybrid thermo‐/opto‐responsive system with simple fabrication, flexibility, rapid response, high sensitivity, and long operational lifetime could pave the way for next‐generation untethered, small‐scale, and intelligent devices.

Carbon dots (CDs), an emerging class of carbon‐based nanomaterials typically smaller than 10 nm, have garnered significant attention over the past decade owing to their excellent aqueous dispersibility, chemical stability, facile surface functionalization, cost‐effectiveness, and unique photoluminescent properties, along with their inherent multifunctionality [6]. Unlike metal‐based nanoparticles, CDs are typically regarded as biocompatible, cost‐effective, and environmentally friendly, making them particularly appropriate for use in biomedical, environmental, and industrial applications [7, 8, 9]. Since the discovery of carbon quantum dots CDs, several types of synthesis approaches have emerged [10]. These methods can be categorized into two primary types: top‐down approaches, which encompass arc discharge, laser ablation, and acidic oxidation, and bottom‐up approaches, which include combustion, microwave‐assisted pyrolysis, solvothermal/hydrothermal synthesis, and electrochemical techniques [11, 12, 13, 14]. However, the tendency of these luminescent nanoparticles to aggregate is considered a limiting factor that can significantly influence their photoluminescence properties. However, the inherent tendency of these luminescent nanoparticles to aggregate represents a critical limitation, as aggregation can significantly alter and often diminish their photoluminescent performance. To address this challenge, the nanoparticles can be incorporated into suitable matrices that effectively suppress aggregation and preserve their optical properties. In this context, the present study develops a fluorescent CD‐integrated, robust nanohybrid thermo‐optical system engineered to deliver advanced thermal‐responsive optical functionality alongside effective infection‐control capability. The integration of CDs with polymer matrices holds significant promise for developing advanced functional materials. The electron‐donating and electron‐accepting nature of CDs allows them to exhibit both antioxidant and pro‐oxidant behavior [14, 15, 16]. Integrating CDs into polymer networks boosts the physicochemical stability of the composite while also providing distinctive optical, electrical, and biological features. Incorporating CDs into polymer matrices enables synergistic enhancement of their intrinsic properties, leading to multifunctional composites suitable for applications including antimicrobial textiles, biomedical scaffolds, biosensing platforms, drug delivery systems, and protective coatings. Compared to the research on the diverse applications of CDs/polymer composites, the investigation of their antioxidant capabilities has received comparatively less attention. Biodegradable polymers of natural origin, including proteins and carbohydrates, offer strong matrix support for biological applications. Due to their safety, sustainability, and eco‐friendly characteristics, bio‐based polymers are increasingly preferred over conventional petroleum‐based materials.

Nosocomial infections, or hospital‐acquired infections (HAIs), represent a considerable worldwide healthcare burden, leading to extended hospitalizations, increased treatment expenses, and higher morbidity and death rates [17]. These infections are mostly induced by pathogenic bacteria, viruses, and fungi that remain on medical surfaces and fabrics, promoting cross‐contamination in clinical environments. Textiles used in hospital linens, uniforms, and protective garments are particularly susceptible to microbial colonization due to their porous architecture and high moisture retention, making them potential reservoirs for infection transmission. Conventional textile finishing approaches frequently rely on chemical agents that raise environmental and health concerns. These treatments often involve heavy metals, toxic compounds, and non‐biodegradable substances that can persist in the environment, bioaccumulate, and pose risks to both ecosystems and human health. Furthermore, commercially available antibacterial fabrics incorporating metallic nanoparticles, while effective, are associated with cytotoxicity and high production costs, limiting their safe and sustainable application [18]. Extended exposure to metal‐based textiles can lead to adverse health effects, including skin irritation, allergic responses, and respiratory complications [19]. To mitigate these risks, the development of alternative active agents for textile treatment is essential, aiming to minimize or eliminate such harmful effects while providing a safer and more sustainable solution. By employing eco‐friendly and non‐toxic materials, it is possible to substantially reduce the environmental impact associated with conventional textile treatments.

Recent advances in functional textiles have extensively explored the incorporation of inorganic nanostructures, metal organic frameworks (MOFs), and fluorescent organic species to impart UV protection, antimicrobial activity, and optical functionality to natural fabrics. For example, Ce‐based mixed metal MOFs immobilized on cotton have demonstrated durable UV shielding with UPF values up to ∼43, while preserving the mechanical integrity of the fabric [20]. Eu‐ and Tb‐based Ln‐MOFs immobilized in cotton demonstrated fluorescent emission along with improved UV protection and antimicrobial properties, especially after cationization of the fabric [21]. Similarly, surface manipulation of dyed silk using silver and palladium salts enabled simultaneous improvement in coloration, UV resistance (UVB blocking ∼97%), and strong biocidal activity [22]. Polymeric functional finishes, such as in‐situ polymerized polybenzoxazines, have also been shown to impart water repellency (contact angle ∼125°) and significant UV protection to cotton fabrics [23]. Self‐implantation of palladium (Pd) nanoclusters within cotton has produced multifunctionality, including exceptional UV shielding with reasonable durability [24], while similar in situ Pd nanoparticle embedding strategies have been extended to viscose fabrics under hydrothermal and infrared‐assisted conditions for long‐lasting UV protection and antimicrobial performance [25, 26]. Silver nanoparticles and AgI immobilized on bamboo fabrics have further demonstrated antimicrobial and photocatalytic activity for wastewater treatment applications [27]. In parallel, several studies have focused on fluorescent organic molecules and heterocyclic compounds to fabricate optically active textiles. Tetrahydrothienoisoquinoline derivatives immobilized on cotton produced highly durable fluorescent fabrics with quantum yields up to ∼68% [28], while quinazoline‐based heterocycles enabled tri‐functional fluorescent, antimicrobial, and UV‐protective textiles for technical and military use [29]. More recently, CDs have emerged as promising candidates for multifunctional textile engineering. CDs nucleated from heterocyclic precursors and immobilized within cotton matrices exhibited fluorescence, UV protection, and microbicidal activity with moderate wash durability [30]. Biopolymer‐blended nitrogen‐doped CDs loaded with iodine have been incorporated into gauze to create antimicrobial and anti‐inflammatory biomedical bandages [31]. CDs‐viscose composite fibers exhibited excellent antibacterial performance with >99.9% bacterial reduction against E. coli and S. aureus when incorporated via wet spinning [32]. CDs hydrothermally derived from Syzygium cumini seed extract were immobilized onto cotton fabrics via an ultrasonication‑assisted process, and the resulting coated textiles retained substantial antibacterial activity, underscoring the potential of biomass‑derived CDs as sustainable functional coatings for antimicrobial biomedical textiles [33]. The incorporation of CDs into textile substrates presents a promising pathway for next‐generation functional fabrics that combine sustainability, biocompatibility, and durable multi‐functional performance for advanced healthcare and environmental applications.

This study presents a simple strategy for incorporating engineered fluorescent CDs‐based composites into cotton and polyester fabrics to produce highly flexible, robust, thermo‐optical textiles with UV‐protective, antimicrobial, and coating‐cycle‐dependent fluorescence properties. Furthermore, these functional textiles were integrated into a mechanically interlocked digital light processing (DLP) 3D‐printing system, enabling the fabrication of complex and stable hybrid architectures while preserving the intrinsic functionality of the coated fabrics. This study includes: (1) the synthesis of CDs via a facile hydrothermal approach using branched polyethylenimine and thymol as precursors. Comprehensive analysis of the CDs’ morphology, structural features, Raman signatures, fluorescence lifetime, and optical properties demonstrated superior photophysical performance, high stability, and strong polar and hydrogen‐bonding interactions, highlighting their potential as nano‐reinforcements in polymer matrices; (2) The formulation of CDs loaded PVA/quaternized chitosan nanocomposites and their deposition onto textiles (cotton, polyester, and surgical masks) using conventional dip‐and‐dry and spray‐coating techniques. To enhance surface functionality, the coated textiles were further modified with hexadecyltrimethoxysilane (HDTMS), where trimethoxysilane groups covalently bonded with surface hydroxyl groups while long hydrophobic alkyl chains reduced surface energy, imparting durable water repellency and liquid resistance. Notably, the functional textiles retained fluorescence even under repeated mechanical deformation, including stretching, folding, rolling, and bending; (3) A systematic dependence of fluorescence response on the number of coating cycles, offering potential utility in wearable sensing, anti‐counterfeiting, and environmental or chemical exposure indicators. Under fixed excitation at 380 nm, CIE analysis showed a marked increase in color purity from ∼24% for CDs to ∼83% after incorporation into the polymer/cotton matrix. This behavior is attributed to coating‐thickness‐dependent optical effects, where thicker films induce self‐absorption, inner filter effects, light scattering, and partial CD aggregation, whereas thinner coatings enable more efficient excitation of surface‐exposed CDs, resulting in stronger and more defined emission; (4) Biological evaluation confirming the non‐cytotoxic nature of the fluorescent textiles alongside strong antibacterial performance against both Gram‐negative and Gram‐positive bacteria, effectively demonstrating infection‐preventive capability; (5) Validation of multifunctionality through UV‐blocking measurements and antioxidant assays, achieving DPPH and ABTS radical‐scavenging efficiencies of 72% and 90%, respectively, with demonstrated retention of performance after repeated washing cycles. (6) Observation of thermo‐responsive fluorescence behavior, where emission intensity increased below body temperature (37°C) and quenched above it. This temperature‐dependent fluorescence, observed under excitation in the 320–420 nm range, highlights potential applications in optical thermometry and smart temperature‐responsive fabrics; (7) Integration of DLP printing directly onto flexible, coated textile substrates to construct hybrid soft–rigid architectures with precise microstructuring. This approach enabled seamless integration of polymeric constructs with flexible fabrics, producing lightweight, durable composites with preserved optical and mechanical performance. Importantly, this strategy not only broadens the material and design possibilities of 3D printing but also establishes a scalable platform for hybrid architectures that unify structural precision, optical functionality, and mechanical compliance. The CDs presented here provide a unique combination of fluorescence, flexibility, UV shielding, antibacterial and antioxidant functionality, coating robustness, and thermo‐responsive optical behavior. To the best of our knowledge, this is the first report demonstrating the integration of mechanically interlocked 3D‐printed soft biomaterials onto CD‐embedded functional textiles, where the textiles deliver thermo‐optical responsiveness, UV protection, and antimicrobial activity, while the printed architectures impart structural adaptability and device‐level integration, together forming a multifunctional intelligent textile system.

2. Materials

2.1. Experimental Section

2.1.1. Synthesis of PTCDs

A simple one‐step hydrothermal method was employed to synthesize the PTCDs. The experimental method is demonstrated as follows: Initially, 0.5 g of branched PEI was added to 30 mL of deionized water. Next, 0.25 g of thymol was dissolved in an alkaline solution and stirred for 1 h. The mixture was then transferred to a poly(tetrafluoroethylene) PTFE‐lined autoclave and heated at 170°C for 11 h. Upon reaching room temperature, a dark suspension was formed. After centrifugation at 10 000 rpm for 45 min, the solution was filtered using a 0.22 µm micropore membrane to remove large particles and subsequently dialyzed. Finally, it was vacuum‐dried and kept at 4°C for further analysis.

2.1.2. Synthesis of Quaternized Chitosan

Quaternized chitosan (QC) was synthesized by reacting chitosan with GTMAC, according to Spinelli et al., with slight modifications [34]. A suspension of 3 g chitosan in 30 mL deionized water was prepared, and 11.29 g (74.4 mmol) GTMAC was dissolved in 15 mL water. After that, GTMAC solution was slowly incorporated into the chitosan suspension, achieving a mole ratio of 4 between GTMAC and the amino groups of chitosan. Following a 24 h reaction at 70°C, the mixture became a turbid yellow solution and was poured into cold acetone. The white product was collected by filtration after multiple precipitation and acetone‐washing steps.

2.1.3. Synthesis of Functional Nanocomposites Coating (FNC)

A simple one‐step hydrothermal method was employed to synthesize the PTCDs. The experimental method is demonstrated as follows: Initially, 0.5 g of branched PEI was added to 30 mL of deionized water. Next, 0.25 g of thymol was dissolved in an alkaline solution and stirred for 1 hour. The mixture was then transferred to a poly(tetrafluoroethylene) PTFE‐lined autoclave and heated at 170°C for 11 hours. Upon reaching room temperature, a dark suspension was formed. After centrifugation at 10 000 rpm for 45 min, the solution was filtered using a 0.22 µm micropore membrane to remove large particles and subsequently dialyzed. Finally, it was vacuum‐dried and kept at 4°C for further analysis.

2.1.4. Surface Treatment of Cotton Textile

The cotton textiles underwent mercerization as a pre‐treatment process [35]. Fabric pieces were immersed in 2.5 M NaOH for 2 min with moderate stirring and subsequently rinsed with running water. The cotton pieces were thereafter immersed in 1% acetic acid for neutralization, followed by multiple rinses and subsequent drying.

2.1.5. Fabrication of the Functional Cotton Fabric

The fabric samples were immersed in the nanocomposite solution (FNC) and were subjected to several coating cycles. A stainless‐steel bowed neck tip holder was used to secure the coated cotton, which was then positioned in an air circulation system. The orientation of the holder was adjusted at regular intervals to ensure a uniform application of coating on the fabric. The samples for the coating cycles 2 and 5 were designated as C2 and C5, respectively. The uncoated fabric has been identified as C0. The nanocomposite solution was uniformly applied onto other substrates, including a surgical mask, and polyester. For the hydrophobic coating, the coated fabric was submerged in a solution containing 3 wt.% HDTMS and thymol in ethanol at room temperature for 5 min, followed by drying at 80°C and curing at 130°C for 5 min.

2.1.6. Antioxidant Activity

A comprehensive account of the materials and methods used in this section is provided in the Supporting Information, including details on synthesis procedures, characterization techniques, and experimental protocols.

2.1.7. Antibacterial Activity

The disc diffusion technique was employed to assess the antibacterial properties of cotton against B. subtilis and E. coli.[36]. The textile samples were cut into squares and sterilized before being carefully positioned on agar plates containing a bacterial suspension of 10⁶ CFU mL⁻¹. After 24 h of incubation at 37°C, the diameter of the inhibition zone free of bacterial growth around the sample was measured. Additional experimental protocols for the MIC and MBC determinations, as well as the time‐dependent antibacterial assays, along with further methodological details, are provided in the Supporting Information.

2.1.8. Temperature‐Dependent Studies

Fluorescence evaluations of the carbon dots and textile samples, influenced by temperature, were performed between 5–100°C using a Cary Eclipse Fluorescence Spectrophotometer. A temperature‐controlled cell holder was employed to sustain thermal conditions throughout the testing.

2.1.9. Fabrication of Mesh‐Like Structures on Fabric Surface via DLP Printing

A digital light processing (DLP) 3D printer (LUMEN X Gen 3, Sweden; operating at a wavelength of 405 nm) was employed for the fabrication of mesh‐like structures directly onto fabric substrates. The photocurable resin (printing ink) was formulated based on our previously reported protocol [37], with a minor modification in the photoinitiator system to optimize polymerization kinetics under the selected printing conditions. For substrate preparation, a double‐sided adhesive tape was applied to the printer stage to ensure stable fixation of the fabric during the printing process. Prior to mounting, the fabric surface was cleaned using a high‐pressure air jet to remove any dust or loose fibers. The fabric was then carefully adhered onto the taped printer stage, ensuring a flat and uniform surface. Following the completion of the printing process, the printed construct was carefully removed from the printer stage and subjected to a post‐print cleaning step. The printed samples were immersed in an isopropyl alcohol (IPA) bath and stirred at 1000 rpm to remove any unpolymerized resin. Finally, the samples were air‐dried under ambient conditions and subsequently analyzed using optical microscopy.

2.1.10. Statistical Analysis

All experiments reported in this study were conducted using a minimum of three independently prepared samples unless otherwise stated. Antibacterial assays, radical scavenging measurements (DPPH and ABTS), UV‐blocking studies, and fluorescence measurements were performed in triplicate, and averaged values are reported. The standard deviations were within acceptable experimental limits, confirming the consistency and reproducibility of the functional textile fabrication process. All results are shown as mean ± SD.

3. Results and Discussions

3.1. Characterization of PTCDs

In this work, PTCDs were synthesized from b‐PEI and thymol through a straightforward hydrothermal method, as shown in Figure 1a. Branched PEI was chosen due to its high density of amine functionalities, which serve as both nitrogen dopants and effective surface passivating groups during carbon dots formation [38, 39]. Nitrogen doping is known to enhance quantum yield and facilitate surface state stabilization, improve electron donating capacity, yielding CDs with high optical stability and strong interaction capability with polymeric hosts such as PVA and quaternized chitosan [40, 41, 42, 43]. The abundance of amine groups also promotes extensive hydrogen bonding, improving coating uniformity, resistance to mechanical deformation, and overall robustness of the CDs/polymer network. These characteristics are important for maintaining fluorescent intensity, UV blocking behavior, and chemical durability during textile bending, stretching, and washing. Thymol was specifically selected as a bio‐derived phenolic compound that contributes to both the optical and biofunctional properties of the CDs. Its aromatic backbone promotes the development of partially graphitic domains within the CD core, improving fluorescence stability and emissive characteristics. More importantly, thymol possesses well‐established intrinsic antioxidant and antimicrobial activities [44, 45]. Thymol's hydroxyl groups participate in condensation and dehydration processes during hydrothermal carbonization, enabling controlled nucleation while simultaneously embedding bioactive functional groups. Incorporating thymol, therefore, allows the CDs themselves to act as multifunctional nano reinforcements, supporting the radical scavenging efficiencies and strong antimicrobial performance observed without requiring external additives. This precursor choice is thus essential for achieving the combined optical, antibacterial, and antioxidant functionality central to the manuscript. This composition enables the resulting CDs to combine intrinsic antioxidant activity with PEI‐derived amine functionality, aligning with the objective of developing metal‐free, multifunctional textiles. TEM was employed as an effective technique to investigate the morphology and structural features of the nanoparticles. TEM image of the PTCDs (Figure 1b) and their size distribution confirm well‐dispersed, quasi‐spherical nanoparticles with an average size of about 5.5 nm. The CDs were uniformly integrated into a polymer matrix and subsequently applied to the textile substrate. The incorporation of CDs resulted in enhanced fluorescence, antimicrobial properties, temperature‐responsive fluorescence, and potential UV‐shielding capabilities, illustrating the textile's multifunctional potential for various applications, including biomedical, protective, and smart textiles (Figure 1c). Digital Light Processing (DLP) 3D printing was used to fabricate precise mesh patterns directly onto the CDs/polymer‐coated textile surface as shown in Figure 1d. This approach enabled high‐resolution deposition of photopolymerizable resins, allowing precise fabrication of three‐dimensional architectures while maintaining the mechanical integrity of the underlying fabric. The printed mesh exhibited strong interfacial adhesion and structural robustness, underscoring the potential of integrating additive manufacturing with functionalized textiles to develop customizable, multifunctional wearable systems and advanced soft‐rigid hybrid devices.

FIGURE 1.

(a) Synthesis of carbon dots (CDs) via a simple hydrothermal method, showing strong fluorescence of their aqueous solution under 365 nm UV light. (b) TEM image of the synthesized CDs. (c) Schematic representation of PTCDs loaded cotton textile, highlighting its multifunctional properties including antibacterial, antioxidant, UV‑protection, and temperature‑responsive fluorescence. (d) Integration of PTCDs coated textile with DLP 3D printing, showing fabrication of patterned mesh structures and their interface with the functional textile.

X‐ray photoelectron spectroscopy (XPS) and Fourier‐transform infrared (FT‐IR) spectroscopy were employed to analyze the chemical composition and surface functional groups of the PTCDs. The FTIR study revealed peaks within the range of 3100–3586 cm⁻¹ are indicative of amine and hydroxyl functional groups, as shown in Figure 2a. The broadening of the peak is ascribed to hydrogen bonding. The peaks observed at 2949, 2808, 1649, and 1425 cm^{− 1} are attributed to C─H and C═O stretching vibrations, respectively, while the band at 1584 cm^{− 1} is assigned to the bending vibration of C─O─NH groups [3, 46]. Additionally, the peaks observed at 1576 and 2363 cm⁻¹ are related to the vibration and deformation bands of N2500H, along with the presence of C≡N, indicating the presence of amino‐containing functional groups [47]. The different peak values at 1452, 1156, and 1053 cm⁻¹ correspond to C─N, C─O─C, and C─O, respectively [48]. The FTIR analysis indicates the existence of –OH, –COOH, and –NH₂ functional groups on the surface of PTCDs. These functional groups enhance the hydrophilicity and aqueous stability of the carbon dots and facilitate their conjugation with polymers and specific biomolecular targets, thereby broadening their potential in biomedical applications. The XPS survey spectrum exhibited three distinct peaks at 283.9, 397.3, and 530.1 eV, corresponding to C 1s, N 1s, and O 1s, respectively, as shown in Figure 2b. The deconvolution of the C 1s spectrum revealed three characteristic components at 287.8 eV (C═O), 284.8 eV (C─O/C─N), and 283.6 eV (C─C/C═C), respectively (Figure 2c) [49, 50]. The N1s spectrum (Figure 2d) displays three different peaks at 396.8, 398.4, and 399.4 eV, which correspond to pyridinic, pyrrolic nitrogen and quaternary nitrogen atoms within the CDs structure, respectively [51]. Furthermore, the high‐resolution O 1s spectrum (Figure 2e) displayed peaks at 530.3, 534.4, and 535.3 eV corresponding to C═O, C─OH/C─O─C, and H─O─H functional groups, respectively [50, 52]. The combined results from FTIR and XPS analyses indicate that the PTCDs are composed of C, N, and O elements. Furthermore, they exhibit a range of bonds and functional groups, such as OH, CH₂, C−C/C═C, C−N and C−O, C═O, and quaternary N. The excellent water‐dispersibility of the carbon dots can be ascribed to the existence of polar functional groups, such as hydroxyl and quaternary nitrogen groups etc., on their surface. Raman spectroscopy of the synthesized PTCDs (Figure S1) reveals two prominent bands centered at ∼1375 and ∼1511 cm⁻¹ , corresponding to the characteristic D and G bands of the carbonaceous nanomaterial, respectively. The D band arises from breathing modes of sp³‐hybridized carbon atoms associated with structural defects, edge sites, and functional groups on the carbon dot surface [53]. In contrast, the G band corresponds to the in‐plane vibrational stretching of sp²‐hybridized carbon atoms in graphitic domains, indicating the presence of ordered graphitic or aromatic structures within the carbon core [54]. The intensity ratio (I_D/I_G) of 1.03 indicates a higher degree of structural disorder relative to graphitic content, which is typical for heteroatom‐functionalized carbon dots where oxygen and nitrogen containing surface functionalities significantly influence their optical and chemical behavior [55, 56].

FIGURE 2.

(a) FT‐IR spectrum of PTCDs. (b) XPS survey spectrum. High‐resolution XPS spectra of PTCDs: (c) C1s, (d) N1s, and (e) O1s. (f) UV–vis absorption spectrum of PTCDs in aqueous dispersion (inset: photographs under daylight, left; and UV light, right). (g) Photoluminescence emission spectra recorded at different excitation wavelengths. (h) CIE chromaticity coordinates at various excitation wavelengths. (i) 3D excitation–emission spectrum and (j) corresponding excitation–emission contour plot. Fluorescence stability of PTCDs measured over (k) different storage periods and (l) under continuous UV irradiation. Data presented as mean ± SD, n = 3.

The optical properties of the PTCDs were investigated using UV‐vis absorption and fluorescence emission spectroscopy. The UV‐vis absorption spectrum of PTCDs, as illustrated in Figure 2f, demonstrates a broad absorption peak at 283 nm. The observed peak can be attributed to n→π* transitions associated with the C═O and C═N bonds [57]. The fluorescence studies of the PTCDs revealed an excitation‐dependent emission behavior. Figure 2g illustrates the shift in the emission peak from 471 to 561 nm as the excitation wavelength was varied from 320 to 460 nm. This phenomenon can be explained by the different sizes of the PTCDs and its varied distribution of surface states that emerge from the distinct functionalities present on the PTCDs' surfaces [58, 59]. The emission intensity reached its maximum when excited at 380 nm. The shift in CIE color coordinates with different excitation wavelengths can be seen in Figure 2h. With the increase in excitation wavelength, the CIE color coordinates of the PTCDs solutions shifted from blue to green. The excitation‐emission survey spectra (Figure 2i) recorded with excitation wavelengths ranging from 300 to 440 nm, along with their corresponding contour plots, are presented in Figure 2j. Figure 2k,l assessed the fluorescence stability of the PTCDs. The fluorescence intensity of the PTCDs shows minimal variation over time, demonstrating their exceptional stability. The fluorescence quantum yield was calculated to be about 16.4% using quinine sulfate in 0.1 M H₂SO₄ as the standard reference. The time‐resolved fluorescence decay of PTCDs (Figure S2) was best fitted using a multi‐exponential decay model, indicating the presence of multiple emissive pathways associated with both carbon core states and surface defect states. The extracted lifetime components were τ₁ = 1.14 ns, τ₂ = 3.85 ns, and τ₃ = 9.07 ns, with a goodness of fit (χ²) of 1.240. The average fluorescence lifetime was calculated to be ∼4.2 ns, reflecting efficient surface passivation and reduced non‐radiative recombination due to nitrogen and oxygen rich functional groups introduced by PEI and thymol. These results confirm that surface‐engineered emissive states play a dominant role in the photophysical behavior of the synthesized carbon dots.

The fluorescence emission spectra shown in Figure 3a demonstrate an evident relationship between PTCDs fluorescence and concentration, varying from C1 to C7 (0.25 to 20 mg mL^{− 1}). Figure 3b shows that aqueous PTCD solutions at different concentrations appear light to dark yellow under visible light and remain stable at ambient temperature. The aqueous solutions of the PTCDs display intense green fluorescence when exposed to UV light (365 nm), as shown in Figure 3c. At lower concentrations, the fluorescence intensity increases progressively, indicating enhanced radiative recombination due to effective dispersion of the PTCDs in the medium [60]. Nevertheless, once a certain concentration threshold is exceeded, a reduction in fluorescence intensity is noted. The observed quenching is related to enhance interparticle interactions arising from the increased density of polar functional groups at higher concentrations. The interactions observed lead to aggregation, as demonstrated in Figure 3d. This process subsequently enables non‐radiative energy transfer pathways and reduces overall fluorescence efficiency [60, 61]. The results emphasize the significant impact of concentration‐dependent colloidal stability on the optical performance of PTCDs. Furthermore, a systematic analysis of the optical properties of the PTCDs was conducted utilizing three‐dimensional excitation‐emission matrices (EEMs). Figure 3e illustrates the EEM spectrum within the excitation range of 320–420 nm (top panel), whereas the bottom panel presents the corresponding contour plot for visualizing the emission intensity distribution. With increasing concentration, the fluorescence intensity peaks at an optimal threshold (Figure 3a), signifying effective emission from well‐dispersed carbon dots. At high PTCD concentrations, the close proximity of fluorescent centers among individual CDs facilitates non‐radiative charge carrier recombination. The increased volume of solvent led to a dilution of the CDs concentration, thereby reducing the non‐radiative relaxation and enhancing the luminescent intensities. However, further dilution reduced the number of fluorescent centers, which contributed to the quenching of intensity. The findings underscore the crucial effect of concentration‐dependent colloidal stability and surface chemistry on the photoluminescence characteristics of nitrogen‐doped PTCDs. The fluorescence properties of the CDs were studied to evaluate temperature effects. As shown in Figure 3f, the fluorescence intensity diminishes with increasing temperature, highlighting a strong temperature‐dependent behavior. The fluorescence intensity as a function of temperature from 15°C to 80°C exhibited a good linear relationship, with a correlation coefficient of R² of 0.99639, as seen in Figure 3g, representing a broader range than previously reported nanothermometers [62, 63]. The mechanism behind the temperature‐dependent properties of carbon dots is still not well‐established. The mechanism behind the response of PTCDs to temperature could be attributed to the increase in the number of non‐radiative channels associated with surface defects as the temperature rises [64]. The temperature‐dependent fluorescence quenching of PTCDs can be ascribed to the activation of non‐radiative pathways associated with surface defects at elevated temperatures. As the temperature increases, enhanced thermal vibrations promote exciton–phonon coupling and facilitate electron relaxation through non‐radiative channels, thereby reducing the overall fluorescence intensity. In contrast, lowering the temperature suppresses these non‐radiative processes, stabilizing excitons in the emissive states and partially restoring the fluorescence intensity [64, 65]. This property enables nanoscale temperature mapping, making CDs useful for biological applications, such as monitoring intracellular temperature fluctuations, and for thermal management in microelectronics. Their biocompatibility and photostability support real‐time, non‐invasive temperature sensing.

FIGURE 3.

(a) Emission spectra of aqueous dispersions of PTCDs at different concentrations (0.25–20 mg mL⁻¹) under identical excitation conditions. (b) Digital images of PTCDs aqueous solutions under daylight, and (c) 365 nm UV irradiation. (d) Schematic illustration of concentration‐dependent fluorescence behavior of PTCDs, along with representative TEM images. (e) Three‐dimensional excitation‐emission matrix (EEM) spectra of PTCDs (top panel) and the corresponding contour plot representation (bottom panel). (f) Temperature‐dependent (10–80°C) fluorescence emission spectra of PTCDs. (g) Linear correlation between fluorescence intensity and temperature.

3.2. Antioxidant Activity

The antioxidant efficacy of the synthesized carbon dots was assessed by two assays: DPPH (2,2‐diphenyl‐1‐picrylhydrazyl) and ABTS (2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid)). CDs, due to their abundant surface functional groups including hydroxyl, carboxyl, and amino groups, have significant radical‐scavenging capabilities. This study employed DPPH and ABTS assays to assess the free radical scavenging ability of the synthesized PTCDs, providing insights into their potential antioxidant applications in biological and environmental contexts. The stability of DPPH as a free radical arises from its nitrogen lone‐pair electrons and three benzene rings. Interaction with an antioxidant converts DPPH to DPPH‐H complex, resulting in a color shift from deep violet to pale yellow [66]. The antioxidant capabilities of PTCDs at all examined concentrations (3.9‐1000 µg mL⁻¹) were assessed using both techniques. The results clearly showed that the PTCDs significantly reduced free radical production in a concentration‐dependent manner. The half maximal effective concentration (EC₅₀) value has been established at 88.6 µg mL⁻¹.The scavenging activity of the DPPH radical increased proportionately with concentration, ranging from 3.9 to 250 µg mL⁻¹ (Figure S3a). The addition of further PTCDs has minimal impact on antioxidant activity. The activity level ultimately attains saturation. The results indicate that PTCDs have significant scavenging activity (about 75%), which varies depending on the dosage. The scavenging effect of PTCDs on ABTS•+ radical increased with concentration, evidenced by the color fading from greenish‐blue to colorless. As shown in Figure S3b, the ABTS·+ scavenging activity increases with the stepwise elevation of PTCDs concentration up to 125 µg mL⁻¹. The introduction of extra PTCDs shows negligible effects on antioxidant activity, reaching a saturation point at approximately 250 µg mL⁻¹. The EC₅₀ value has been established at 37.1 µg mL⁻¹. The improved ABTS scavenging by PTCDs can be attributed to their water solubility and uniform dispersion in the aqueous medium. The findings are consistent with DPPH‐based measurements, indicating a lower free radical scavenging capacity. This reduction is likely due to limited interactions between PTCDs and DPPH radicals in methanol [67].

The surface functionalities of oxygen and nitrogen are considered crucial in the scavenging of reactive oxygen species [46, 68]. Studies suggest that electron‐donating doped sites and sp² carbon domains serve as the main active sites for radical scavenging, facilitating free radical neutralization through adduct formation and spin delocalization across the conjugated carbon network [69, 70]. Dopants enhance radical scavenging by providing electrons to radicals, resulting in proton transfer or adduct formation [16, 71]. The ROS scavenging ability of PTCDs arises from hydrogen‐donating functional groups at the edges, such as –OH, –COOH, and –NH₂, combined with electron delocalization and stabilization within the sp² carbon framework.

Numerous studies have demonstrated that CDs derived from biomass, plant extracts, and heteroatom‐doped precursors exhibit radical scavenging ability owing to the presence of oxygen and nitrogen‐rich surface functional groups and conjugated sp² carbon domains that facilitate electron and hydrogen transfer to reactive radicals [72, 73, 74, 75]. To evaluate the antioxidant performance of the synthesized PTCDs, we compared their radical‐scavenging efficiencies with representative carbon dot systems reported in the literature (Table S1). In this context, PTCDs exhibit competitive antioxidant capacity with EC₅₀ values of 88.6 µg mL^{− 1} (DPPH) and 37.1 µg mL^{− 1} (ABTS), indicating strong antioxidant performance when compared with many reported carbon dot systems in the literature. These values are superior to many reported CDs derived from glucose, citric acid, graphene‐based systems, and several biomass sources, while remaining comparable to more specialized antioxidant‐focused CDs [72, 76, 77, 78]. ABTS scavenging performance of PTCDs falls within the high‐performance range reported for nitrogen‐ and oxygen‐functionalized CDs. Although certain biomass and plant extract‐derived CDs optimized solely for antioxidant activity may demonstrate lower EC₅₀ values, these systems are typically not designed for multifunctional applications [74, 79]. Importantly, the PTCDs reported here are not exclusively optimized for antioxidant activity but are engineered for multifunctional textile applications, combining antibacterial activity, UV‐blocking capability, thermo‐responsive fluorescence, and coating robustness. Despite this multifunctional design constraint, the antioxidant performance remains highly competitive with literature benchmarks.

3.3. Fabrication and Characterizations of Functional Cotton Fabric

A stepwise dip‐and‐dry approach was used for coating, and the fabric was mercerized to enhance its surface polarity prior to treatment. After mercerization, the activated fabric was immersed in a nanocomposite coating solution and then dried in a hot air oven until completely dry. This process allowed for uniform deposition of the nanohybrid layer on the cotton fibers, facilitated by hydrogen bonding and polar‐polar interactions among the hydroxyl groups of cellulose, the amino groups of chitosan, and the hydroxyl functionalities of PVA and carbon dots, as illustrated in the interaction scheme (bottom left of Figure 4). To impart hydrophobicity, the coated fabrics were further treated with a solution of hexadecyltrimethoxysilane (HDTMS) in ethanol. The coated cotton sample was dipped in the HDTMS solution and then cured at 130°C, facilitating the condensation of the methoxy groups of HDTMS with the surface hydroxyls of cellulose, therefore forming covalent Si─O─C bonds. The chemical modification effectively lowered surface energy, resulting in a highly water‐repellent fabric without compromising their functional properties. Figure 4 illustrates the schematic representation of the procedure used to prepare the functional textile samples. The nanocomposite solution was applied to different substrate materials, including cotton fabric, surgical mask layers, and polyester textiles, to assess its coating efficacy and functional integration across diverse types of commonly used biomedical and protective fabrics. The proposed methodology provides a safe, cost‐effective, and environmentally friendly alternative to traditional metal‐based composite technologies in functional and medical textiles.

FIGURE 4.

Schematic representation of the fabrication of coated cotton fabric using a consecutive dip‐and‐dry method. The coating components are shown on the left. Bottom left: Interaction between CDs, polymer chains, and the cellulosic units of cotton.

FTIR and XPS analyses were performed on both untreated and coated cotton fibers. Figure 5a presents the FTIR spectra of the cotton fabrics prior to and following coating. The FTIR spectrum of quaternized chitosan is presented in Figure S4, with the corresponding discussion provided in the Supporting Information. The spectrum of untreated cotton displays the characteristic absorption bands of pure cellulose. The FTIR spectrum of pristine cotton exhibits characteristic absorption peaks at 3323, 2905, and 1642 cm⁻¹, corresponding to the stretching vibrations of O─H, C─H, and C═O bonds, respectively [80]. In addition, the distinct bands in the range of 900–1552 cm⁻¹ is observed, which is attributed to C─H, O─H, C─O, and C─O─C stretching vibrations of the cellulose backbone [81, 82]. The stretching vibrations of the C–O–C bond in the β‐d‐glucoside linkage are associated with absorption bands found at about 898 and 1158 cm⁻¹ [81]. The FTIR spectrum of the coated cotton reveals distinct cellulose bands, accompanied by additional features resulting from the CDs/polymer coating. The broadening of the peak at 3339 cm⁻¹ in the FTIR spectra of the composite textile signifies hydrogen bonding interactions between the −OH groups of the PTCDs and polymer matrix and the cellulose building blocks of cotton. The coated fabric exhibited a noticeably broadened and slightly red‑shifted O─H/N─H stretching band (3600–3000 cm⁻¹) compared to pure cotton, which is characteristic of strengthened hydrogen‑bonding networks among cellulose hydroxyls, PVA chains, chitosan amine/─OH groups, and the surface functionalities of the CDs. The C─N derivatives display distinct peaks between 1328 and 1557 cm^─1, offering compelling evidence for the existence of the nanocomposite coating on the cotton fabric [46]. In addition, the appearance of Amide I (∼1650 cm^{− 1}) and Amide II (∼1550 cm^{− 1}) bands confirms the presence of chitosan within the composite layer, while the band near ∼1500 cm⁻¹ corresponds to the quaternary‑ammonium (N⁺(CH₃)₃) groups of quaternary chitosan, indicating the availability of cationic sites. The enhanced C‐O‐C/C─O stretching region (∼1160–1020 cm^{− 1}) further verifies deposition of the polymeric layer on the cellulose fibers. Subsequent hydrophobic treatment with HDTMS reveals notable spectrum changes. The reduced intensity of O─H stretching at around 3360 cm^─1 indicates a decrease in surface hydroxyl groups resulting from silane grafting and the disruption of hydrogen bonds. The characteristic absorption peaks of the Si─O─Si bonds in the HDTMS within the 1100−1000 cm⁻¹ range are overlapped by the cellulose bands resulting from C─O bending modes [83].

FIGURE 5.

(a) FTIR spectra of cotton, coated cotton, and coated cotton with hydrophobic finishing. (b) XPS survey spectra of pristine (uncoated) and coated cotton. (c) High‐resolution C1s scan of pristine cotton (uncoated). (d–f) High‐resolution XPS scans of coated cotton: (d) C1s, (e) N1s, and (f) O1s regions. SEM images of (g) uncoated cotton fabric, (h) C2, (i) C5, and (j) coated cotton fabric after hydrophobic finishing (HC).

To gain deeper insight into the surface chemical modifications induced by the coating process, XPS analysis was performed on the cotton fabric before and after FNC coating. The corresponding XPS survey spectra are presented in Figure 5b. The XPS spectra of pristine cotton display notable peaks for C and O at 283.0 and 530.3 eV, respectively. The appearance of a distinct N peak at 397.7 eV, alongside the characteristic C and O signals, provides clear evidence of successful composite textile fabrication. This nitrogen contribution can be attributed to the incorporation of PTCDs and/or the chitosan polymer matrix, both of which introduce nitrogen‐containing functional groups detectable in the XPS spectra. The high‐resolution C1s spectrum of pristine cotton (Figure 5c) exhibits three characteristic peaks at 284.7, 286.3, and 288.7 eV, which can be assigned to C═C/C─C, C─O, and C═O groups, respectively. In contrast, the C1s spectrum of the coated cotton (Figure 5d) shows notable changes, with peaks at 284.5 eV (C═C/C─C), 285.6 eV (C─N/C═N), and 287.5 eV (C═O) [84]. These findings indicate the formation of new C─N/C═N functionalities, supporting the successful surface modification.

The N 1s spectrum (Figure 5e) shows three distinct peaks at 398.8, 400.0, and 401.9 eV, corresponding to C─N, C═N and protonated/quaternary species, respectively [84, 85]. The O1s spectra of the uncoated cotton are shown in Figure S5. The high‐resolution O 1s spectrum of the coated cotton (Figure 5f) reveals a new peak at 532.1 eV, attributed to C═O functionalities, in addition to the signals at 531.0 and 533.7 eV, which correspond to O─C═O and C─OH/C─O─C groups, respectively [86, 87]. These observations support the successful incorporation of nitrogen‐containing moieties during the coating process. The observed spectrum changes indicate the successful modification of the cotton surface, leading to enhanced functional qualities for diverse applications.

SEM images of both untreated and coated cotton fabrics provide a detailed view of surface morphology and the impact of the coating process. Figure 5g–j depicts structural alterations as the number of coating cycles increases. The uncoated cotton fabric (Figure 5g) exhibits the typical fibrous structure of cotton, with loosely arranged fibers, smooth surfaces, and distinct strands forming a porous network. Following the application of a two‐coating cycle (C2), as illustrated in Figure 5h, the cotton fibers demonstrate compaction and reveal the presence of a thin coating layer. This coating has a little smoothing effect on the fiber surfaces and partially occupies the interstitial spaces between the fibers. This effect becomes increasingly evident at higher magnifications. With the application of five coating cycles (C5), as shown in Figure 5i, the coating layer is more pronounced and substantially alters the fabric's morphology. The fibers are almost entirely covered by the polymer coating, forming a thick, continuous, and uniform layer. At lower magnification, the coated fabric appears considerably more compact and less porous than the previous samples. Higher magnification images reveal extensive and uniform coverage, with only minimal visibility of the underlying fiber structure. The surface exhibits a smooth, film‐like appearance, indicating that repeated coating cycles have effectively generated a robust protective layer on the cotton fibers. After hydrophobic finishing (HC) (Figure 5j), the coated cotton fibers appear fully covered by a homogeneous layer, with a slightly roughened surface that can enhance water repellency. SEM imaging confirms successful coating of cotton fibers, with the coating thickness and uniformity increasing from C2 to C5.

3.4. Mechano‐Physical Properties of the Functional Cotton Fabric

To evaluate the multifunctionality of the coated fabrics, we investigated both their mechanical properties and their ultraviolet (UV) shielding performance. The representative stress–strain curves (Figure 6a) show that the coated fabrics (C2, C5, and C5H) maintain the characteristic deformation profile of pristine cotton (PC), indicating that the coating process does not compromise the textile's inherent flexibility. Quantitative analysis of the ultimate tensile strength (UTS) and elongation at break (EB) (Figure 6b) confirms only marginal variations between coated and uncoated fabrics, with UTS consistently above 25 MPa and EB in the range of 10%–14%, thereby demonstrating that the coatings do not deteriorate the load‐bearing capacity of the fabrics. Likewise, measurements of elastic modulus and toughness (Figure 6c) reveal stable values across all formulations, underscoring that the structural reinforcement provided by the coating is achieved without embrittlement or loss of energy absorption capability. In contrast to the preserved mechanical properties, a marked enhancement in optical functionality is observed.

FIGURE 6.

(a) Typical stress‐strain plot of the uncoated and coated fabrics. (b) Ultimate tensile strength (UTS) and elongation at break (EB) of the fabric samples before and after coatings. (c) Elastic moduli and toughness of the fabric samples before and after coatings. (d) Transmittance plot of the uncoated fabric and coated fabrics. (e) Schematic illustration of the proposed mechanism by which CDs‐coated fabrics enhance protective performance. (f) Relative increment in blocking (RIB) of UV_A and UV_B of the fabrics showing the performance of CDs based coating formulation. (g) TGA thermograms of the uncoated cotton and coated cotton fabrics. (h) Magnified portion of TGA plot of different specimens showing the onset of degradation and impact of coating on thermal stability of the fabrics. (i) Magnified image of the char (residue) formation at high temperature. (j) Contact angles of the uncoated cotton fabric and other coated fabrics. Left side shows the different surface polarities and interactions with water molecules. Hydrophobic chains after silane functionalization shows water repelling affect. Data presented as mean ± SD, n = 3.

3.5. UV‐Protective Behaviors

UV‐vis transmittance spectra (Figure 6d) show that uncoated cotton readily transmits UVA and UVB radiation, whereas the coated fabrics exhibit significantly suppressed transmittance, particularly in the 200–400 nm region. This attenuation is most pronounced for the higher concentration (C5) and C5H coatings, indicating superior shielding efficiency. The UV‑shielding performance of the coated textiles was quantified using UPF analysis (AATCC TM 183–2010). Pure cotton exhibited a UPF of ∼34, indicating very good protection, and the C2 modified fabric showed a similar UPF (∼34). In contrast, the C5 coating significantly enhanced UV blocking, achieving UPF 50+ (59.3), which corresponds to excellent protection and a substantial reduction of both UVA and UVB transmittance compared to the uncoated and lightly coated samples.

The schematic illustration in Figure 6e depicts the proposed mechanism, in which the CDs‐based coatings act as an optically active layer, absorbing and scattering incoming UV radiation while maintaining transparency to visible light and allowing radiative heat transfer, thereby ensuring comfort and usability. The quantitative measure of UV‐blocking efficiency, expressed as the relative increment in blocking (RIB), is presented in Figure 6f. Both UVA and UVB blocking efficiencies increase systematically with coating concentration and post‐treatment, with the C5H sample exhibiting the highest performance, surpassing 95% RIB in the UVB region. This demonstrates that the CDs‐based coating formulation transforms conventional cotton into a multifunctional fabric with intrinsic UV protection while retaining mechanical robustness and comfort. Such a combination of durability and optical selectivity is highly desirable for wearable applications, particularly in protective clothing, outdoor textiles, and healthcare‐associated fabrics where simultaneous mechanical reliability and UV shielding are essential.

3.6. Thermal Stability Analysis

Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability and decomposition behavior of both uncoated and coated cotton fabrics. The complete TGA curves for pristine cotton (PC), C2‐coated cotton, and C5H cotton samples are illustrated in Figure 6a. All specimens demonstrate a significant degradation phase occurring between 300°C and 400°C, which aligns with the thermal breakdown of cellulose, the main constituent of cotton fibers [88]. The uncoated cotton shows a pronounced and abrupt weight loss, indicating rapid thermal degradation and limited char‐forming ability. In contrast, the coated samples, particularly the C5H variant, exhibit delayed degradation onset and a broader decomposition profile, reflecting enhanced thermal stability imparted by the surface coatings [89]. Figure 6b provides an enlarged perspective of the initial degradation phase (400–460°C), emphasizing the beginning of thermal decomposition. The data clearly indicate that the temperature at which weight loss begins increases for coated fabrics, with the C5H showing the most significant postponement. The thermal shielding effect can be linked to the protective barrier characteristics of the hybrid coating, which has the potential to restrict oxygen diffusion and heat transfer to the underlying cellulose matrix, thus improving its thermal durability. The diminished slope observed in the initial degradation phase of the coated fabrics provides additional evidence of a more gradual decomposition process, indicating the efficacy of thermal insulation offered by the surface‐modified layers. Additionally, Figure 6c focuses on the high‐temperature area (exceeding 500°C) to evaluate the residue or char yield following the primary decomposition event. The uncoated cotton exhibits the least residual mass, whereas both coated samples, especially the C5H variant, maintain notably greater char content. The increase in char residue indicates enhanced carbonaceous structure formation facilitated by the coating, which serves as a flame‐retardant barrier and encourages the development of thermally stable residues. The increase in char yield indicates improved fire resistance and suggests the possible use of these coated fabrics in thermal protection and flame‐retardant textile applications [90]. The TGA results clearly indicate that the surface coatings, particularly those with hydrophobic properties, significantly enhance the thermal stability and char‐forming ability of cotton fabrics.

3.7. Surface Wettability

Wettability and surface polarity of both uncoated and coated cotton fabrics were evaluated using static water contact angle measurements, as shown in Figure 6j. The pristine cotton exhibited an almost imperceptible contact angle, reflecting its naturally hydrophilic characteristics. This is attributed to the high concentration of surface hydroxyl (‐OH) groups present in cellulose, which readily form hydrogen bonds with water molecules [91]. Upon coating with a dual‐component system (C2 and C5), there was a gradual increase in contact angle, reaching ∼40°–60°, confirming partial reduction in hydrophilicity due to the surface modification [92]. Nonetheless, these values indicate surfaces that are moderately wettable, highlighting the existence of polar functionalities and a minimal change in surface energy. A notable increase in hydrophobicity was achieved for the HC sample, where the cotton fabric underwent coating and subsequent functionalization with Hexadecyltrimethoxysilane (HDTMS). The contact angle rose significantly to exceed 100°, indicating a shift from a hydrophilic to a distinctly hydrophobic surface. The significant enhancement in water repellency can be linked to the surface grafting of long‐chain alkyl silane molecules [93]. The right side of the figure illustrates that unmodified cotton surfaces possess hydroxyl groups capable of forming hydrogen bonds with water. In contrast, the silane‐modified surface features densely packed alkyl chains, which greatly reduce surface energy and interfere with water–surface interactions. The hydrolysis and condensation reactions of HDTMS with the cellulose ─OH groups lead to the formation of covalent Si─O─C linkages. Meanwhile, the hydrophobic hexadecyl chains extend outward, creating a barrier that repels water. The transformation of the surface from polar to non‐polar domains significantly inhibits water spreading and promotes droplet beading, which is a defining characteristic of superhydrophobicity. The gradual change in contact angle under different coating conditions highlights the combined effect of chemical modification and hierarchical structuring on the fabric's wettability. The modification of hydrophobic properties significantly enhances moisture resistance, self‐cleaning capabilities, and the performance of outdoor textiles, positioning HDTMS as a highly effective functional silane for providing lasting water‐repellent characteristics to materials derived from natural fibers.

3.8. Evaluation of Coating Robustness

We assessed the durability of the coated fabric against two common stressors: laundering and prolonged thermal exposure. High‐resolution SEM images collected at progressively higher magnification show that the coating remains continuous and conformal on individual filaments and yarn crossovers after a standard wash (Figure 7a–c). The fiber bundles retain their native twist and spacing, pore openings between yarns are preserved, and there is no evidence of coating detachment, particulate shedding, or fiber‐to‐fiber blocking. The magnified images reveal well‐defined filament outlines with continuous coverage, indicating that the coating adheres to and conforms to the textile microtopography rather than forming a brittle layer. Extended heat treatments produced similarly stable morphologies. After continuous exposure at 80°C for three days (Figure 7d–f), the weave architecture and filament striations are indistinguishable from the washed specimen, with no detectable crack networks, ridge formation, or interfacial gaps along filament boundaries. Elevating the stress to 100°C for 24 h (Figure 7g–i) does not introduce thermal wrinkling, sintering, or melting artifacts; the coating still follows the filament contours and maintains open inter‐yarn porosity. Even at 130°C for 24 h (Figure 7j–l), the micrographs reveal intact edges at yarn crossovers and smooth, continuous coverage on single filaments. Across all conditions, we do not observe delamination fronts, craze marks, or debris fields that would signal cohesive failure of the coating or adhesive failure at the fabric–coating interface. These results indicate that the coating forms a mechanically compliant and thermally stable interphase with the textile, capable of withstanding agitation, surfactant exposure, and prolonged heating without loss of microstructural integrity. Such robustness is essential for real‐world use, where textiles face repeated washing, drying, and occasional sterilization steps. In our broader platform, this stable coating is also the key tie layer that enables reliable add‐on processing, including 3D printing of microstructured features onto fabrics, because it preserves pore geometry, maintains breathability, and provides a durable surface for subsequent patterning. The invariance of the SEM signatures from Figure 7a–l supports long service life and consistent performance under practical operating conditions.

FIGURE 7.

Coating robustness of the functional fabric after washing and thermal treatments. (a–c) High‐resolution SEM images of the coated fabric after washing, along with corresponding magnified views, demonstrating preservation of surface integrity. (d–f) SEM images of the fabric after continuous thermal treatment at 80°C for 3 days, showing no noticeable surface degradation. (g–i) SEM images after continuous heat treatment at 100°C for 24 h, indicating no significant changes at the fabric surface. (j–l) SEM images following continuous heat treatment at 130°C for 24 h, confirming the structural stability of the coating.

Figure 8a displays digital photographs of cotton fabrics under daylight, with C0, C2, C5, and HC representing untreated cotton, FNC coated samples with two, and three coating cycles, and coated cotton after hydrophobic coating respectively. Figure 8b,c depicts the fabric in various physical forms, including bent and rolled conditions. No signs of coating detachment, fragmentation, or chalking were observed, indicating that the nanocomposite forms a robust and mechanically stable layer on the textile surface. In addition to cotton, the nanocomposite was also applied to polyester substrates. The uncoated and coated polyester samples are displayed in Figure 8d. Under UV illumination, the coated polyester fabrics exhibit strong fluorescence, confirming the presence of the coating (Figure 8e). Furthermore, hydrophobic finishing of the coated polyester samples demonstrates the effectiveness of the treatment (Video S1). Digital images of the nanocomposite coating under daylight and UV illumination are shown in Figure 8f,g, respectively. The nanocomposite retains its optical properties over extended periods, demonstrating its suitability for practical applications. Upon exposure to UV light, the material displays noticeable fluorescence properties. The high surface energy of the nanoparticles encourages their inherent inclination to aggregate. The polymer phase acts as a stabilizing agent by adsorbing to the nanoparticle surfaces, leading to the formation of stable nanocomposites. This underscores the suitability of the material for functional textile applications that require long‐term stability and consistent optical performance.

FIGURE 8.

(a) Digital images of cotton samples C0 (untreated), C2, C5, and HC. Dip coated cotton sample in (b) bend and (c) rolled position. (d) Dip coated polyester sample before and after coating under daylight. (e) Fluorescence image of the same polyester sample under UV irradiation (365 nm). (f) Digital images of the coating nanocomposite under daylight, and (g) UV light. (h) Finger‐rubbing test with rough nitrile glove tips under daylight and UV light showing no chalking or residue from the coated textile surface. (i) Scotch tape adhesion test performed under daylight, demonstrating no fluorescent particles transferred to the tape, indicating the coating's mechanical robustness. (j) image of the spray bottle used for coating. Digital images of cotton fabrics before and after spray coating: (k,l) untreated cotton under daylight and UV light, respectively; (m) cotton after nanocomposite spray coating, showing uniform coverage and strong fluorescence under UV light in bend and rolled position. (n) Digital photographs of coated cotton after exposure to water, hot water, and SDS for 5 and 20 min, observed under visible light (top rows) and UV illumination (bottom rows). (o) Digital images of liquid droplets on untreated cotton (left) and hydrophobic cotton fabric (right), illustrating the difference in wettability between the two surfaces. (p) Digital images of coated cotton with hydrophobic finishing, captured under UV light at different time intervals.

Coating robustness is a key non‐destructive measure of fabric durability. Two methods were employed for the durability assessment. Nitrile gloves with rough fingertips were utilized for the finger rubbing test and rubbed repeatedly. Figure 8h illustrates the various stages of the finger rubbing test, demonstrating that there is no chalking of the coating materials from the fabric surface (Video S2). Furthermore, to ensure this, the entire procedure was also observed under UV light, as illustrated in Figure 8h under UV, verifying the absence of fluorescent fragments (Video S3). Similarly, the coated specimen was tested using pressure‐sensitive adhesive tape, which was applied to the fabric and repeatedly peeled off, as shown in Figure 8i. Analogous to the finger rubbing test, no fragment pieces were observed throughout this examination. Video monitoring (Video S4) of the entire process under UV illumination further supports this observation. Alongside the dip‐coating treatment, we investigated spray‐coating as an alternate methodology for applying the nanocomposite onto textile substrates (Figure 8j). The untreated cotton fabric prior to spray coating, observed under daylight and UV illumination, is shown in Figure 8k,l, respectively. As shown in Figure 8m, after spray coating, the cotton fabric exhibits uniform coverage and strong fluorescence under UV light, confirming successful deposition of the nanocomposite. The surface morphology of the spray‐coated cotton sample is shown in SEM images (Figure S6). Figure 8m further shows the spray‐coated cotton fabric in various physical forms, including bent and rolled, seen under both daylight and ultraviolet lighting. The coated cotton retains its fluorescent characteristics despite undergoing twisting or mechanical deformation. This flexibility is essential for practical applications in wearable technology and smart textiles, where the material must withstand various forms of physical stress while maintaining its structural integrity. This approach offers advantages such as uniform coverage, reduced material consumption, and the potential for scalable, industrial fabrication of functional fabrics. The durability of the CDs/polymer coating was assessed by immersing the coated cotton samples (HC) in water, hot water, and SDS solutions for varying durations (5 and 20 min), and thereafter observing them under both visible and UV light. As shown in Figure 8n, no fluorescent fragments or leached nanoparticles were found in the surrounding media, signifying that the carbon dots remained firmly embedded inside the polymer matrix on the cotton surface.

Cotton is among the most widely used fabrics for everyday clothing, medical textiles, hospital linens, home décor, and kitchen applications due to its comfort and breathability. However, its inherently porous structure allows it to absorb water, moisture, and oils, making it prone to staining from common substances such as food, beverages, and blood, which can limit its practical utility. In this work, we focus on not only imparting hydrophobicity to cotton fabrics but also enhancing their resistance to stains from typical household liquids, including water, milk, coke, green tea, coffee, turmeric solution, red, and blue dyes. For all tested liquids, the untreated cotton fabric readily absorbed the droplets upon contact, consistent with its intrinsic hydrophilic nature and porous structure (Figure 8o, left). In contrast, the hydrophobic cotton (HC) fabric exhibited a markedly different behavior, where the liquid droplets remained on the surface in well‐defined spherical shapes with minimal spreading (Figure 8o, right). This pronounced repellency toward a wide range of liquids highlights the successful modification of the cotton surface to achieve hydrophobicity. Such behavior not only confirms the effectiveness of the surface treatment but also suggests potential advantages for applications requiring water resistance, stain repellency, or self‐cleaning properties. The Videos S5 and S6 demonstrates the behavior of oil droplets on pure cotton and on HC sample. The HC sample with hydrophobic coating was immersed in water, and its fluorescence was monitored over several days. Digital images of the glass vial containing the hybrid textile (Figure 8p), captured under UV light, show that the fluorescence remained largely unchanged even after 15 days. To assess the washing durability of the UV‑protective function, we compared the UV‑radiation transmittance of the fabrics before washing and after three washing cycles. The transmission profiles show only minor increases in UV transmittance after washing, indicating that the coating remains largely intact and continues to suppress UV passage through the fabric (Figure S7). This confirms that the added UV‑blocking functionality is stable under repeated washing, supporting its suitability for practical wearable applications. Compared with previous studies in which UV‑protective cotton was fabricated through the direct deposition of metals such as Ag, Au, Zn, Cu, or Ti onto textile substrates [94, 95], the UV‑shielding performance of the coated cotton samples are noticeably higher. Comparable levels of protection have been reported only for MOF‑based textile treatments [96]. These findings collectively demonstrate that the present CDs based functionalization approach achieves enhanced efficiency in imparting durable UV protection to cotton fabrics.

3.9. Thermo‐fluorescence Behavior of Functional Cotton Coatings

A series of photoluminescence studies of the coated cotton were conducted. The photoluminescence spectra of the PTCDs/polymer‐coated cotton fabric at varying excitation wavelengths (300–440 nm) are presented in Figures S8,S9. Both the C2 and C5 fabrics showed maximum emission at ∼445 nm under the studied excitation wavelengths. Interestingly, compared to pristine CDs, which show a maximum emission at ∼503 nm under 380 nm excitation (Figure 2), the coated textile exhibits a distinct blue‐shifted emission (∼445 nm), indicating modification of the surface states by the polymer/cotton microenvironment [97]. This shift can be attributed to polymer‐induced surface passivation, which minimizes non‐radiative recombination, as well as to confinement effects imposed by the cotton matrix that reduce electronic conjugation. The 3D excitation–emission survey spectra and the corresponding contour plots of C2 and C5 are presented in Figure 9a–d. The HC sample exhibits identical absorption edges and retains the same maximum emission profile (∼445 nm) (Figure S10) across the studied excitation wavelengths compared to the samples before thermal exposure (Figure S8), confirming that the core structure and surface passivation of the CDs remain unaffected by the processing temperature. These results indicate that the short thermal exposure during the hydrophobic coating (130°C) does not induce structural degradation or alteration of the emissive surface states, thereby preserving the intrinsic optical properties of the CDs within the textile matrix.

FIGURE 9.

Excitation–emission characterization of CDs and coated cotton fabrics. (a, c) 3D excitation–emission survey spectra of C2 and C5, respectively; (b,d) corresponding excitation–emission contour plots. CIE chromaticity coordinates of (e) pristine CDs, (f) C2, and (g) C5; (h) calculated color purity of CDs, C2, and C5 samples. (i) Digital fluorescence images of coated cotton fabrics under different excitation wavelengths. (j) Temperature‐dependent photoluminescence (ΔPL) response of the functional cotton. (k) Top: 3D excitation–emission survey spectra of coated cotton at varying temperatures (5°C, 15°C, 37°C, 50°C, 75°C, and 100°C); bottom: corresponding excitation–emission contour plots.

The CIE chromaticity coordinates of the CDs, C2, and C5 are shown in Figure 9e–g, respectively. The pristine CDs exhibit coordinates at (x = 0.238, y = 0.395), corresponding to greenish emission, while C2 (x = 0.150, y = 0.086) and C5 (x = 0.153, y = 0.103) are located in the deep‐blue region of the CIE diagram, indicating a significant blue shift in emission upon coating. At a fixed excitation of 380 nm, the CIE analysis revealed a marked increase in color purity upon incorporation of the CDs into the polymer/cotton matrix. The pristine CDs exhibited relatively broad green‐cyan emission centered at 503 nm, corresponding to a low color purity of 24.25%. Such broad emission is typical for carbon dots due to their heterogeneous surface states and multiple emissive centers, which produce overlapping transitions and therefore reduced chromatic saturation [98, 99]. In contrast, the 2‐coating cotton displayed blue emission at 443 nm with significantly enhanced color purity (83.20%), while the 5‐coating cotton showed a comparable emission peak at 445 nm but slightly reduced purity (78.58%) (Figure 9h). The pronounced enhancement in color purity for the coated fabrics can be attributed to two factors: (i) surface passivation and confinement effects imparted by the polymeric matrix, which stabilize specific emissive states of the N‐doped CDs and suppress competing defect‐related transitions, leading to spectrally narrower emission; and (ii) selective reabsorption filtering in the composite environment, which preferentially attenuates longer‐wavelength tails, thereby shifting the emission toward the blue region with greater spectral purity. Fluorescence images of C2 recorded at different excitation wavelengths ranging from 340 to 600 nm are shown in Figure 9i. The slight reduction in color purity observed in the 5‐coating sample compared to the 2‐coating can be attributed to increased self‐absorption and inner‐filter effects arising from the thicker coating layers. Analogous behavior has been reported in nanocomposite systems: for instance, Hahm et al. observed fluorescence quenching when quantum dots were placed too close to silica‐coated metallic cores, a direct result of thickness‐dependent interactions [100]. Additionally, Tang et al. documented pronounced quenching through the inner‐filter effect in carbon dot–based sensors, where overlapping absorption attenuates emission intensity [101].

The PL emission spectra demonstrate the temperature‐dependent fluorescence behavior of the coated cotton fabrics. Under 380 nm excitation, as the temperature increases from 5°C to 100°C, a progressive reduction in PL intensity is observed. The emission, centered at ∼445 nm, is most pronounced at lower temperatures (5°C and 15°C), whereas significant thermal quenching occurs at elevated temperatures, particularly above 50°C. Figure 9j shows the variation of photoluminescence (ΔPL) of CDs/polymer‐coated cotton as a function of temperature. The data indicate a distinct temperature‐dependent fluorescence response relative to physiological body temperature (37°C). The coated cotton shows a positive ΔPL at temperatures below body temperature, indicating increased fluorescence intensity as temperature decreases. This implies that CDs inside the polymer matrix become more emissive at lower thermal energy conditions, most likely due to limited non‐radiative relaxation pathways and restricted molecular vibrations. In contrast, when the temperature is increased beyond body temperature, the ΔPL values shift to the negative region, reflecting a progressive quenching of fluorescence. The coated cotton exhibits a thermo‐responsive fluorescence behavior: fluorescence intensity increases below 37°C and decreases above 37°C. This dual response demonstrates the potential of CDs/polymer‐coated cotton in temperature‐sensitive optical sensing and biomedical applications, using body temperature as a natural reference point.

The reduction in emissions at higher temperatures can be linked to increased thermal agitation and potential structural changes at the polymer/CDs interface, which contribute to suppressing radiative emission. This quenching effect can also be attributed to enhanced non‐radiative relaxation pathways at elevated temperatures, where increased thermal vibrations promote exciton–phonon interactions, thereby reducing the efficiency of radiative recombination [102]. Moreover, the polymer matrix may undergo structural relaxation with increasing temperature, altering the microenvironment around the CDs. This can lead to reduced confinement or changes in hydrogen‐bonding interactions, both of which can affect the fluorescence signal.[97]. In addition, nitrogen‐doped CDs are known to exhibit temperature‐sensitive photoluminescence due to surface state transitions and thermally activated trap states, which become more prominent at elevated temperatures, further contributing to emission quenching [103]. The incorporation of biopolymers such as chitosan and synthetic PVA can partially stabilize the photoluminescence through hydrogen bonding and enhanced matrix rigidity. However, with increasing temperature, thermal motion likely perturbs these interactions, leading to emission quenching. The 3D emission survey spectra and corresponding contour plots coated cotton at different temperatures are shown in Figure 9k and Figure S11. Temperature‐dependent fluorescence intensity allows these fabrics to serve as wearable indicators of body or environmental heat, highlighting their potential applications in healthcare monitoring, safety apparel, and sportswear. The thermo‐fluorescent responsive behavior can be applied in anti‐counterfeiting, interactive fashion, and educational demonstrations, underscoring the multifunctional potential of these engineered textiles. This temperature‐dependent fluorescence behavior highlights the potential of such nanocomposites for applications in optical thermometry and temperature‐sensing textiles [104].

3.10. Antioxidant and Antibacterial Activity of the Functional Textile

3.10.1. Antioxidant Activity

The significance of antioxidant textiles has grown steadily, driven by their potential health benefits and wide‐ranging applications across various industries. These textiles are designed to counteract oxidative stress, a condition resulting from an imbalance between free radicals and antioxidants in the body. This imbalance leads to cellular damage and plays a role in aging and various disorders. Textiles loaded with antioxidants have the potential to neutralize free radicals present on the skin's surface, which may reduce the risk of skin conditions and improve overall skin health. Moreover, antioxidant textiles offer an additional protective barrier against external contaminants and harmful UV radiation, particularly in environments where individuals are exposed to high levels of oxidative stress, such as critical care units or during extended hospital stays. The antioxidant efficacy of the CDs/polymer nanocomposite FNC and the corresponding coated cotton samples was methodically assessed utilizing two recognized radical scavenging tests: the DPPH and ABTS assays. Both techniques are extensively utilized for measuring radical scavenging capability. DPPH radicals undergo reduction upon hydrogen atom donation, leading to a drop in absorbance. On the other hand, ABTS•+ radicals are neutralized by antioxidants, leading to a notable color change and a reduction in absorption. The DPPH radical scavenging activity of the coating nanocomposite was evaluated by measuring the absorbance following 30‐minute incubation in the dark at various concentrations.

As shown in Figure 10a, the coating nanocomposite demonstrated a dose‐dependent rise in DPPH radical scavenging efficiency, achieving around 60% activity at higher doses (1000 µg mL⁻¹). Assessment of DPPH radical scavenging for nanocomposite‐coated cotton was carried out after 30 and 60 min in the dark. Scavenging activity, depicted in Figure 10b, increased with the number of coating cycles, reaching a maximum of ∼74% for C5. A parallel trend was noted in the ABTS assay (Figure 10c,d), indicating that both the nanocomposite and the coated cotton exhibited significant radical cation scavenging activity, surpassing 90–95% at elevated concentrations or coating levels. The results demonstrate that the nanocomposite exhibits robust antioxidant properties, effectively neutralizing a substantial quantity of ABTS radicals, even at low concentrations. The rapid attainment of a plateau indicates that the nanocomposite's active sites saturate rapidly, enabling maximal radical scavenging even at low concentrations. The composite textiles exhibit higher ABTS•+ antioxidant activity with increasing coating cycles, as illustrated in Figure 10d. Color changes from blue‐green to colorless in FNC‐coated cotton confirm significant ABTS•+ radical scavenging, with scavenging activities of ∼90.2% for C2 and ∼98% for C5. In contrast, uncoated cotton (C0) exhibited minimal activity in both DPPH and ABTS assays. These results confirm that the coating effectively imparts antioxidant properties to the fabric, making it suitable for diverse applications. The durability of the antioxidant functionality of the coated textiles was evaluated by monitoring ABTS radical scavenging activity after repetitive washing cycles (Figure S12). The HC sample exhibited ∼97% ABTS scavenging efficiency prior to washing. After one washing cycle (W1), the scavenging activity remained high, and even after three washing cycles (W3), the fabric retained ∼85% of its initial antioxidant performance. The marginal reduction in activity indicates that the PTCDs/polymer nanocomposite coating remains strongly adhered to the cotton fibers, preserving the accessibility of active oxygen and nitrogen‐containing functional groups responsible for radical neutralization. Such retention is comparable with other reported antioxidant textiles. For example, textiles treated with propolis‐honey biosolutions maintained high initial antioxidant activity but still demonstrated performance reduction after 3 washing cycles, regardless of the mordant presence [105]. Similar results were also observed by Meena et al. for the modification of knitted cotton fabric using pomegranate peel waste [106]. Reports specifically evaluating the wash durability of antioxidant functionality in coated or finished textiles remain relatively scarce in the literature, with most studies focusing primarily on initial antioxidant performance rather than retention after repeated washing.

FIGURE 10.

Assessment of antioxidant activity: DPPH radical scavenging activity of (a) coating nanocomposites and (b) different coated textiles, measured after 30 and 60 min incubation in the dark. ABTS radical cation scavenging activity of (c) coating nanocomposites and (d) coated textiles under identical conditions. Bottom panel: schematic representation of the antioxidant mechanism in the functional textile. Zone of inhibition (ZOI) evaluation of untreated and coated cotton against (e,g) E. coli and (f,h) B. subtilis, respectively. Antibacterial activity of uncoated and coated samples against (i) E. coli and (j) B. subtilis following incubation in liquid culture and subsequent colony growth on agar plates. Data are presented as mean ± SD, n = 3.

Figure 10e depicts the radical scavenging mechanism of the functional textile, in which the nanoparticles act as hydrogen donors to neutralize DPPH and ABTS•+ radicals, thereby inhibiting oxidative degradation. Dopants may help the radical scavenging process by donating electrons to radicals, resulting in proton transfer or adduct formation [71]. Collectively, our findings indicate that FNC‐coated cottons exhibit strong and dose‐dependent antioxidant properties. The observed strong activity can be attributed to the combined effects of PTCDs and thymol. The HDTMS finishing does not inhibit activity; rather, it facilitates the immobilization of thymol and stabilizes the underlying nanocomposite layer, thus maintaining radical scavenging capability after the hydrophobic modification. The capacity to incorporate high‐performance radical scavenging characteristics into functional textiles highlights the potential of this approach for improved healthcare and protective material applications.

3.10.2. Antibacterial Activity

Textiles with combined antioxidant and antibacterial properties are essential in clinical settings, as they improve patient safety and help reduce the incidence of healthcare‐associated infections (HAIs). By suppressing the growth and proliferation of pathogenic microorganisms, such functional textiles help lower infection risk and promote better healthcare outcomes. The antibacterial properties of these textiles help limit the growth and dissemination of pathogens on medical garments, bedding, and wound dressings, thereby promoting a safer and more hygienic environment for both patients and healthcare personnel. Antioxidant textiles can scavenge free radicals, supporting cell regeneration and reducing skin damage, ultimately contributing to improved patient outcomes. In this study, we evaluated the antibacterial activity of the synthesized PTCDs against both Gram‐negative Escherichia coli (E. coli) and Gram‐positive Bacillus subtilis (B. sub). The samples were evaluated for their inhibitory capacity by the minimum inhibitory concentration (MIC) test. The antibacterial efficacy of the PTCDs samples was evaluated by exposing bacterial cultures, prepared at a density of 10⁶ CFU mL⁻¹, to the nanoparticles under controlled conditions. The MIC was measured according to the previously reported procedure [107]. MIC corresponds to the minimum sample concentration required to inhibit visible bacterial growth following overnight incubation. Various concentrations of PTCDs were tested against diluted bacterial suspensions in this study. After 24 h of incubation at 37°C, the minimum inhibitory concentration (MIC) was obtained by identifying samples exhibiting no apparent bacterial growth. The CDs effectively reduced the growth of both E. coli and B. subtilis, with MIC values found at 0.06 and 0.03 mg mL⁻¹, respectively. To identify the minimum bactericidal concentration (MBC) of the PTCDs, aliquots from samples showing no visible growth in the MIC assay were subcultured onto fresh agar plates and incubated at 37°C for 24 h. The minimum bactericidal concentration was established as the lowest concentration of nanoparticles that led to complete elimination of bacterial colony formation. The MBC values of the PTCDs were determined to be 0.125 and 0.06 mg mL⁻¹ for E. coli and B. subtilis, respectively. This approach allowed the evaluation of the bactericidal activity of the CDs against E. coli and B. subtilis, thereby complementing the MIC results and offering a comprehensive evaluation of their antibacterial efficiency. A comparative analysis of antibacterial performance, based on recent literature, is provided in Table S2. Previous studies have established that the antibacterial performance of carbon dots is strongly governed by their surface chemistry, charge, and nanoscale dimensions [108]. Travlou et al. demonstrated that sulfur and nitrogen co‐doped carbon dots exhibit enhanced antibacterial activity against E. coli and B. subtilis due to the presence of surface amide and amino groups, whose protonated forms promote strong electrostatic interactions with negatively charged bacterial membranes, ultimately leading to membrane disruption and cell death [109]. These findings emphasize that surface functionalization, rather than composition alone, dictates the nano‐bio interactions responsible for antibacterial action. The positive zeta potential of the PTCDs promotes electrostatic attraction toward the negatively charged bacterial surfaces, enabling close contact at the nano‐bio interface. The amine‐rich functionalities originating from branched PEI, along with phenolic groups contributed by thymol, facilitate membrane destabilization by disrupting the lipid bilayer and interfering with membrane integrity. This interaction increases membrane permeability, leading to leakage of intracellular components and eventual bacterial cell death. The observed antibacterial efficacy of PTCDs against both Gram‐negative and Gram‐positive strains is therefore attributed to charge‐driven membrane disruption and surface functional group mediated interactions rather than metal‐based toxicity or external biocidal agents.

The antibacterial activities of functional textiles were evaluated through the disc diffusion test against both gram‐negative and gram‐positive bacteria. When a substance has enough antibacterial capabilities to prevent bacterial growth, a distinct, clear area free of bacteria, known as the zone of inhibition (ZOI), appears. The presence of this zone suggests that the substance successfully inhibits the growth of bacteria. Due to the nutrient‐rich characteristics of natural cotton fibers, untreated cotton often shows an absence of antibacterial properties and may even promote bacterial proliferation. Consistent with this, our studies showed that untreated cotton textiles exhibited no detectable zone of inhibition (ZOI). The ZOI for the different cotton samples coated with nanocomposites (C2 and C5) is presented in Figure 10e,f, demonstrating the superior antibacterial efficacy of the treated fabrics in comparison to the untreated cotton. Increasing the number of coating cycles resulted in higher deposition of nanocomposites on the cotton surface, thereby enhancing its antibacterial activity. These results highlight the effect of nanocomposite deposition on bacterial growth inhibition and show the progressive enhancement of antibacterial efficacy with increasing coating cycles. The antibacterial activity of cotton textiles was also evaluated after coating with an HDTMS‐based hydrophobic layer. After the hydrophobic coating, the textiles retained antibacterial activity, indicating that the HDTMS layer did not compromise their antibacterial function. The antibacterial activity was retained after the hydrophobic modification, as demonstrated by comparable ZOI values. Similar antibacterial trends were also observed in the FNC‐coated mask samples. The untreated mask (UM) exhibited no ZOI, suggesting a lack of inherent antibacterial activity. In contrast, masks treated with the nanocomposite demonstrated notable antibacterial properties, which increased with the number of coating applications (Figure S13).

The durability of the antibacterial effect was evaluated following several washing cycles with water, hot water, and detergent solutions containing SDS. The coated cotton demonstrated significant antibacterial performance across all washing conditions, reflecting a robust retention of the functional coating. The coated cotton showed a substantial decrease in bacterial colony compared to uncoated control fabrics, as shown in Figure S14. Figure S15 illustrates the antibacterial performance of HC‐coated cotton fabrics against E. coli and B. subtilis after three washing cycles, evaluated by liquid culture incubation followed by agar plate colony assessment. The control samples exhibit dense bacterial growth for both strains, indicating normal proliferation in the absence of the coating. In contrast, the HC fabric shows few scattered colonies, demonstrating good retention of antibacterial activity even after repeated washing. These results confirm the wash durability and sustained bactericidal efficacy of the HC‐coated textile. The antibacterial results are comparable with previously reported studies, indicating that functionalized textiles maintain sustained bactericidal efficacy following multiple wash cycles, confirming the feasibility of achieving wash‐durable antibacterial coatings on cotton substrates. For instance, Emam et al. reported that CQDs nucleated from pyrimidine‐based heterocycles and immobilized on cationized cotton retained 63%–68% microbial inhibition against E. coli, S. aureus, and C. albicans even after repeated washing cycles, while maintaining UV‐protective and fluorescent properties, demonstrating wash‐resistant microbicidal performance [30]. Similarly, Sharmoukh et al. described advanced technical textiles incorporating quinazoline‐based heterocyclic compounds that exhibited sustained fluorescence, UV shielding, and antibacterial efficacy after repeated laundering, highlighting the feasibility of durable multifunctional textile finishes for practical applications [29]. The endurance results from the covalent connection between HDTMS and cotton, the physical entrapment of thymol within the siloxane network, and hydrogen‐bonding interactions, which collectively inhibit the leaching of active components during washing. Figure 11a illustrates the time‐dependent antibacterial activity of the nanocomposite‐coated mask on nutrient agar plates. Analysis of bacterial cultures exposed to the nanocomposites at varying incubation times reveals that the nanocomposite‐coated mask effectively inhibits substantial bacterial growth within a few hours. In contrast, the control plates for both bacterial strains show substantial colony growth over the same period, as shown in Figure 11. Although a few colonies are visible on the 24 h C2H‐M and C5H‐M plates, their number is negligible compared to the extensive growth observed in the control. A similar trend was observed for nanocomposite‐coated polyester samples, which also exhibited substantial reduction in bacterial growth as depicted in Figure 11b and Figure S16.

FIGURE 11.

Time‐dependent antibacterial activity against (a) E. coli and B. subtilis strains of masks, and (b) polyester substrates. (c) Schematic illustration of the corresponding antibacterial experiment. UM: untreated mask; C2‐M: mask with two coating cycles; C5‐M: mask with five coating cycles; C2H‐M and C5H‐M: C2‐M, and C5‐M samples after hydrophobic finishing. UP: untreated polyester; C2‐P: polyester with two coating cycles; C2H‐P: C2‐P sample after hydrophobic finishing. (d) Schematic illustration of the proposed antibacterial mechanisms.

The synthesis of CDs was conducted through a hydrothermal reaction using branched polyethyleneimine as a precursor. PEI‐functionalized nanomaterials exhibit extensive applications in antibacterial treatments, attributed to the presence of primary, secondary, and tertiary amino groups on the PEI surface [110]. These functional groups confer excellent water solubility and strong antibacterial activity against both Gram‐positive and Gram‐negative bacteria [111]. The QDs can infiltrate bacterial membranes, potentially disrupting intracellular components and resulting in bacterial cell death [111]. Multiple hypotheses have been suggested to explain the antibacterial mechanisms of CDs, which are affected by surface functional groups, electrostatic interactions, particle size, and concentration, as well as reactive oxygen species generation. Prior research indicates that the antibacterial effectiveness of nanoparticles is significantly affected by variables including surface charge, surface functionalization, and structural properties [107, 108]. Surface amide linkages and amino groups on CDs are crucial for their antibacterial activity. In their protonated form, these groups interact electrostatically with bacterial membrane phospholipids, inducing physical damage and cytoplasmic leakage, ultimately resulting in cell death [112]. The antimicrobial mechanism of CQDs involves penetration into bacterial cells, disruption of metabolic pathways through cell wall binding, and alteration of cellular signaling. Effective antimicrobial activity relies on the initial uptake of agents by the bacterial cell wall, either through direct contact or electrostatic interactions. Once the cell wall is penetrated, this can cause cytoplasmic leakage, protein degradation, chromosome condensation, and disruption of DNA replication. Additionally, the distribution of nanoparticles within bacterial membranes is highly dependent on particle size, with very small nanoparticles penetrating the cell wall more easily. Consequently, the ultrasmall CDs can efficiently penetrate microbial cell walls, thereby enhancing their antibacterial activity (Figure 11d).

PVA functions as the polymer matrix in the nanocomposite, facilitating a continuous film that allows for uniform deposition onto the cotton substrate during the coating process. The film‐forming capability and hydrophilic properties facilitate robust interactions with the fiber surface and ensure effective adhesion of active components. PVA not only serves as a matrix but also enhances the mechanical stability and flexibility of the coated fabric by preventing cracking or peeling of the functional layer during handling or use. Quaternized chitosan (QC) serves as an additional polymer matrix that offers structural and functional benefits. The cationic quaternary ammonium groups of QC enhance electrostatic interactions with negatively charged bacterial cell walls, thus imparting intrinsic antibacterial activity. Furthermore, QC interacts with PVA and the active nanocomponents, which improves the dispersion of CDs within the matrix. PVA and QC form a robust, multifunctional nanocomposite coating that integrates mechanical durability, efficient loading of active agents, and improved antibacterial performance on the textile substrate. HDTMS functions primarily as a hydrophobic modifier, creating a crosslinked siloxane network that decreases surface wettability and restricts bacterial adhesion. The retention of activity can be attributed to the hydrophobic nature of HDTMS, which reduces bacterial adhesion and proliferation on the fiber surface. Furthermore, the crosslinked siloxane network formed by HDTMS could contribute to the retention of the underlying antibacterial coating, thereby preserving its effectiveness. Liu et al. reported that HDTMS‐treated cotton exhibits excellent anti‐adhesive properties, effectively preventing the majority of bacteria from attaching to the fabric surface [113]. The presence of thymol in the system further contributed to the overall antibacterial activity, thereby enhancing the efficacy of the functional coating. The experimental findings underscore the crucial significance of anti‐adhesion efficacy in the material's antibacterial action. These results suggest that HDTMS coatings can provide water‐repellency while preserving the antibacterial properties of the textile.

Alongside antibacterial activity, cytotoxicity is a crucial factor that affects the use of any material in both industrial and medical applications. Fibroblast proliferation was observed on all substrates over a period of 1–3 days. Pure cotton facilitated substantial cell growth, whereas C2 and C5 coated samples demonstrated comparable or slightly improved proliferation, thereby confirming the biocompatibility of the coatings as shown in Figure S17. The proliferation of fibroblast cells in culture wells was also observed over a period of 1 to 3 days for untreated cotton and samples coated with C2 and C5 (Figure S18). All samples supported cell adhesion and proliferation, with untreated cotton showing good biocompatibility.

3.10.3. Printing Over Coated Fabric

3D printing directly onto flexible, coated textile substrates is an emerging approach for creating hybrid soft–rigid architectures with advanced functionalities. Figure 12a illustrates the schematic of digital light processing (DLP) printing of a thin mesh layer onto a coated fabric surface, facilitated by double‐sided adhesive fixation. The complete 3D printing process on the fabric is demonstrated in Video S7. The digital photographs of the fabricated construct under ambient daylight, side view, and ultraviolet (UV) illumination clearly demonstrate the strong adhesion of the printed structure to the fabric and reveal the fluorescent emission of the coated textile, in contrast to the non‐fluorescent nature of the printed mesh. This selective fluorescence further highlights the successful integration of dissimilar materials without interference in their intrinsic optical properties. Microscopic characterization confirms the high‐resolution pattern fidelity achievable on the coated textile substrate. As shown in Figure 12b, the printed mesh is precisely formed on a 2 cm × 2 cm coated fabric specimen, exhibiting uniform periodic features. A magnified image of the mesh (Figure 12c) reveals the fine grid structure with well‐defined boundaries, while the tilted view (Figure 12d) emphasizes the layered growth of the printed object. Further high‐magnification views (Figure 12e–f) establish the high degree of printability (>92%), as evidenced by the well‐defined pores and layer continuity. Microscopy videography of the 3D printed mesh on the fabric surface is provided in Video S8. Importantly, cross‐sectional imaging of the fabric–printed mesh interface (Figure 12g) shows intimate contact and interpenetration between the printed layers and the textile surface, ensuring robust adhesion and mechanical stability during and after printing. The ability to directly print microstructured patterns onto coated fabrics using DLP technology offers significant potential for the development of next‐generation wearable devices, smart textiles, and bio‐integrated systems. Unlike conventional approaches that rely on post‐print attachment or adhesive lamination, this method establishes seamless integration between polymeric constructs and flexible substrates, enabling durable, functional, and lightweight composites. This strategy not only broadens the material and design possibilities for 3D printing but also offers a scalable approach for fabricating hybrid platforms that integrate mechanical flexibility, optical functionality, and high structural precision. The interfacial chemistry between the fabric and printed polymer phase is also illustrated in Figure 12 h. The adhesion between the fabric and the printed polymer phase could be attributed to polar‐polar interactions between fabric cellulose chains and the printed polymer‐CDs polar functionalities. Moreover, during the initial stages of printing, the polymer inks tend to penetrate and fill the interstitial spaces within the fabric. This allows the ink to achieve a larger surface area in contact with the fabric, leading to mechanical interlocking between the printed base layers and the exposed fabric surface.

FIGURE 12.

(a) Schematic of DLP printing of thin layer mesh onto the coated fabric surface showing adhesion and different digital photos at different ambiances viz. daylight, side view, and under 365 nm UV exposure. Under UV exposure showing fluorescence of the fabric but non‐fluorescence nature of the printed mesh. (b) Optical microscopic images of the printed mesh on a 2 cm × 2 cm coated fabric specimen. (c) Magnified image of the printed mesh. (d) Tilted view of the printed mesh showing layers. (d,f) Magnified image of the mesh showing >92% printability. (g) Fabric‐printed mesh interface showing the layers onto the fabric during DLP printing. (h) Schematic illustration of the interfacial chemistry between cotton fabric and the printed polymer phase.

4. Conclusion

This study presents the development of a fluorescent CDs based nanocomposite formulation for textile coatings, featuring multifunctional properties including strong fluorescence, UV protection, antibacterial activity, and temperature‐responsive optical behavior. The single‐step hydrothermal synthesis of CDs and their incorporation into biopolymer matrix facilitated the thin, coherent coating of a large variety of fabrics, not limited to cotton, polyester, and surgical mask materials via scalable dip‐and‐dry and spray techniques. Surface modification with hexadecyltrimethoxysilane imparted durable liquid repellence while preserving optical activity, even after repeated mechanical deformation. The fluorescence intensity demonstrated a response that depended on the number of coating cycles, serving as an inherent indicator for sensing applications. The multifunctional textiles demonstrated strong antibacterial activity, notable radical scavenging capability, and tunable thermo‐optical emission, enabling potential applications in optical thermometry and smart textile‐based sensors. Additionally, direct DLP printing onto the coated fabrics allowed the fabrication of intricate, mechanically interlocked 3D hybrid structures while preserving both optical and mechanical integrity. Together, these findings provide a versatile and scalable framework for developing smart fabrics that combine precise structural design, mechanical flexibility, and dynamic optical functionality. This method might be a suitable cost‐effective alternative for advanced wearable technologies, hybrid systems that combine soft and rigid elements, and bio‐integrated structures aimed at real‐time sensing and environmental observation.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File 1: adhm71043‐sup‐0001‐SuppMat.docx.

Supporting File 2: adhm71043‐sup‐0002‐VideoS1.mp4.

Supporting File 3: adhm71043‐sup‐0003‐VideoS2.mp4.

Supporting File 4: adhm71043‐sup‐0004‐VideoS3.mp4.

Supporting File 5: adhm71043‐sup‐0005‐VideoS4.mp4.

Supporting File 6: adhm71043‐sup‐0006‐VideoS5.mp4.

Supporting File 7: adhm71043‐sup‐0007‐VideoS6.mp4.

Supporting File 8: adhm71043‐sup‐0008‐VideoS7.mp4.

Supporting File 9: adhm71043‐sup‐0009‐VideoS8.mp4.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.